This invention relates to a force detector, and more particularly to a force detector for detecting forces applied or exerted in respective axial directions and moments applied or exerted about respective axes in an XYZ three-dimensional coordinate system wherein the origin is used as a working point.
Generally, devices for detecting a force exerted on a working point are constructed to detect a stress-strain produced by the application of this force to thereby indirectly make a force detection. The detection of a stress strain is carried out by providing sensors such as strain gauges at respective portions of a strain generative body which produces a strain-stress by the application of a force, and measuring changes in resistance values, etc. of these detectors.
Recently, there has been proposed a technique to arrange, on a semiconductor substrate, resistance elements having the property of the piezo resistance effect that an electric resistance varies by mechanical deformation to detect a force from changes in the resistance values of these resistance elements. This technique intends to cause a mechanical deformation on the semiconductor substrate by the application of force, thus to electrically detect changes in the resistance values of the resistance elements produced by such a mechanical deformation. For example, there is disclosed, in the International Application based on the Patent Cooperation Treaty No. PCT/JP88/394, a technique to form a plurality of resistance elements at predetermined positions along the X-axis and Y-axis directions on a substrate extending on the XY-plane and assemble these resistance elements into a peculiar bridge circuit, thereby making it possible to detect forces applied or exerted in respective axial directions and moments exerted about respective axes as changes in the bridge voltage.
However, the force detector disclosed in the above-described International Application has the following problems.
(1) In order to allow a mechanical deformation to be easily produced on a substrate on the basis of an applied force, several portions of the substrate are hollowed out to form a first bridge portion along the X-axis direction and a second bridge portion along the Y-axis direction. Where an attempt is made to bond this substrate to a strain generative body comprised of metal, etc., an approach is employed to mount a bonding agent or adhesive on the surface of the stain generative body to melt this bonding agent under the condition of a high temperature to mount the substrate thereon and make a bonding between the strain generative body and the substrate. However, since the substrate includes hollowed out portions therein, the molten bonding agent permeates through the hollowed portions to appear on the substrate surface. The bonding agent which has appeared on the substrate surface in this way exerts a bad influence on various wiring implemented on the substrate surface. This is not favorable.
(2) Fluctuation in the bridge portion is apt to occur. As a result, there are instances where correct measured values cannot be obtained.
(3) Detection of six components of forces in three axial directions of XYZ and moments about three axes thereof can be made, but there are instances where any other component may interfere with a measured value of a component subject to measurement, failing to obtain a correct measured value.
(4) At the time of detecting forces or moments of six components, there occurs a considerable difference between the detection sensitivities of respective components. In actual terms, the detection sensitivity of Fx or Fy becomes equal to about one tenth of the detection sensitivity of Fz. Namely, there is a decuple difference between the detection voltage value when a force is applied in the Z-axis and that when the same force as above is applied in the X-axis or Y-axis. For this reason, it is difficult to precisely process respective output signals.